The present invention relates to a process for the selective removal of the impurities, e.g. arsenic, antimony, selenium, tellurium and bismuth, present in substantially sulfidic complex and/or mixed ores and concentrates or technical precipitates which contain similar minerals, by breaking up and rearranging, at an elevated temperature and at a high pressure of elemental sulfur, the minerals present in the raw material, in order to cause the new impurity compounds produced in the rearranging to pass into the gas phase.
The present invention relates in particular to a process for the removal, before the metallurgical refining of the principal metals, of metals which are to be regarded as impurities in relation to the principal metals present in primarily sulfidic complex and mixed ores and concentrates. The bulk of these elements, which are bound in the sulfides of copper, nickel, cobalt and iron as complicated and stable complex structures, consist of arsenic, antimony and bismuth. The scope of the invention also covers a large number of elements which independently form complex minerals or lie in the lattices of others. Such elements include Se, Te, Ga, In, Tl, Ge, Sn, Pb, Zn, Cd, Hg, Mo, Mn, Re, Ag, and Au.
Arsenic, antimony and bismuth cause very great problems in the metallurgy of copper and nickel. In pyrometallurgical processes the compounds of these components, being easily dissociable to metals, are carried along throughout the processing of the principal metals. Efforts are made during each process stage to remove these components, since, when left in the raw metal, they complicate the purification of the raw metal and, when left in the final product even in very small concentrations, they lower the grade of the product.
The impurities contemplated are usually stacked with the principal metal as complicated complex compounds, and therefore the pretreatment of the ore or concentrate by vaporization annealing or, for example, selective froth-flotation does not produce results. Neither are processes for the selective leaching of the components successful, either for the above reasons or for thermodynamic reasons due to the impurity metals themselves.
In the production of copper by conventional processes (reverberatory smelting, sulfide conversion, electrolysis), part of the arsenic, antimony and bismuth present in copper concentrates can be removed. However, in order to obtain a satisfactory final result, purification operations must be included in each process stage. This naturally leads to difficult and uneconomical treatments of solid and molten phases, to large quantities of intermediate products, and, respectively, to the production of circulating loads which limit the capacity of the equipment.
Continual attempts have been made to improve the techniques of removing the impurities at various process stages. In connection with the production of sulfide matte the removal of the impurity components under discussion can be influenced by a suitably selected smelting technology. In shaft, reverberatory and electric-furnace smelting, approx. 50% of the said impurities present in the feed remain in the sulfide phase. In suspension processes, especially in the production of sulfide mattes rich in valuable metals (strong suspension oxidation), the results obtained are considerably better than those mentioned above, especially as regards arsenic and bismuth. Some examples of the suspension processes are the processes according to U.S. Pat. Nos. 3,754,891, 2,506,557, 3,555,164, and 3,687,656 and the processes analogous to them.
Development over recent years has made it possible to increase the separation of the impurities under discussion per apparatus at the conversion stage from the conventional values (70-75%) to values above 90%. The separation has been improved by, for example, combining the impurities, by oxidizing them, by using alkali or iron oxides, to form stable compounds which can be separated in a melt. Owing to the mixing conditions and other conditions, these commonly used processes are costly and their efficiency is low. For example, metals As, Sb, Bi, Pb, Zn, Fe, Co and Cu can be removed quantitatively from the sulfide melts following the conversion of iron. The development over recent years in this area is illustrated by, for example, the processes for the chlorination of nickel (U.S. Pat. No. 3,802,870) and copper (U.S. Pat. No. 4,054,446) sulfide melts.
From the nickel sulfide melt the impurities Fe, Co, and Cu, for example, can be removed by extraction with a chloride mixture melt (750.degree.-900.degree. C.). Sufficiently pure nickel can be blasted directly at a high temperature from the melt (Ni.sub.3 S.sub.2) obtained as a product.
In attempts at removing impurity metals (Zn, Bi, Pb, Sb, As) from a copper sulfide melt (1150.degree.-1200.degree. C.) by halogenation, the activity of copper in the melt must be lowered in order to prevent the copper from being chlorinated, by adjusting the composition of the melt to the sulfur-rich side of the Cu-Cu.sub.2 S solubility gap. Simultaneously the activity of the impurity metals increases, and their selective halogenation becomes possible. In carrying out the process, the absence of a gap in the solubility of the sulfide and chlorine of copper is of considerable importance for its kinetics.
The halides of many heavy-metal impurities are thermally so stable that, for example, at temperatures of 1600.degree.-1800.degree. C. a large quantity of impurities can be halogenated into the gas phase from melts containing Cr-CO, Ni-Fe (U.S. Pat. No. 4,006,013). In this case the activities of the principal metals must also be lowered, in order to prevent halogenation, by adjusting the quantity of carbon in the melt and, when necessary, also the hydrogen pressure in the system. In the pre-purification of the raw metal in an anode furnace the same techniques are used as in the conversion. It should be noted that in metal melts the activity condition of the impurity metals under discussion are very disadvantageous, and so the removal of impurities which have passed into the metal melt is highly uneconomical by current methods.
Halogenation processes have been subjected to a great deal of research also as regards the recovery of the principal metals of solid sulfidic and oxidic ores in the form of halides, for example, for hydrometallurgical refining. The halogenation of the principal components from sulfides at 600.degree.-700.degree. C. is not, however, very selective. The mechanism of the processes is slow and inhibited (the sulfides have surface layers consisting of molten and solid halides). The following publications describe the fundamentals of halogenation: H. H. Kellogg: J. of Metals, Trans. AIME, 188, 1950, 862; R. Richte: Die Thermodynamischen Eigenschaften der Metallchloride, VEB Verlag Technik, Berlin, 1953; J. Gerlach, D. Papenfuss, F. Pawlek, R. Reihlen: Erzmetall, XXI, 1968, 9; J. K. Gerlach, F. E. Pawlek: Trans. AIME 239, 1967, 1557; R. J. Fruehan, L. J. Martonik: Met. Trans. 4, 1973, 2789-2797. The very recent separation processes, worth mentioning, based on the halogenation of metals include the separation of the nickel and copper of silicate and ultrabasic ores by the so-called segregation roasting or its derivatives (an alkali or earth-alkali chloride and carbon halogenation-reduction system): A. A. Dor: The Metallurgical Society of the American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc., New York 1972, 1-310.
The purification of the products of the roasting of pyrites and sulfurous pyrite, consisting of a great number of various methods, should also be mentioned. The object is to remove the sulfur, arsenic and antimony, as well as valuable metals, from the calcine, in which case the treated calcine is a suitable raw material for iron production. The processes are usually one- or two-stage oxidation and reduction processes, nearly always involving a sulfating, chlorinating or vaporizing roasting. Fluidized-bed furnaces are generally used for the implementation of the processes. The processes according to U.S. Pat. No. 3,649,245 and Canadian Pat. Nos. 890,343, 876,030, 885,378 and 882,585 are examples of the latest state of the art.
The object of the present invention is to provide a process for a more selective and more economical vaporization of the impurities present in complex and mixed ores.